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Thread: ArrayList implementation

  1. #1
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    Default ArrayList implementation

    Hi,

    I need to do the following.

    Suppose I have a class A and class B and C are subtypes of A.

    I want to implement an ArrayList that contains subtypes of A. But I only want to implement this ArrayList using methods in class A.

    I was thinking about using the bounded wildcard, but I could not get it to work.

    So, I need something like:

    public class MyArrayList<? extends A> extends ArrayList<? extends A> {
     
        /**
         * Return a subtype of A
         */
        @Override
        public <? extends A> get(int index) {
            // do something with methods only in A.
        }
     
        /**
         * Add a subtype of A.
         */
        @Override
        public boolean add(<? extends A> e) {
            // do something with methods only in A
        }
     
    }

    Also I want to override the methods get and add in ArrayList. So when I use MyArrayList, I expect to do the following.

     
    MyArrayList<B> bList = new MyArrayList<B>();
    MyArrayList<C> cList = new MyArrayList<C>();
     
    bList.add(new B());
    B b = bList.get(0);
     
    cList.add(new C());
    C c = cList.get(1);

    I could not get this to work. Any ideas how this should be done? Or is this not a good design?


  2. #2
    Crazy Cat Lady KevinWorkman's Avatar
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    Default Re: ArrayList implementation

    Can't you just use an ArrayList of A?

    ArrayList<A> list = new ArrayList<A>();
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    Default Re: ArrayList implementation

    Quote Originally Posted by KevinWorkman View Post
    Can't you just use an ArrayList of A?

    ArrayList<A> list = new ArrayList<A>();
    Then how will the get method now whether to return an object of the type B or C?

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    Default Re: ArrayList implementation

    Quote Originally Posted by meijuh View Post
    Then how will the get method now whether to return an object of the type B or C?
    It won't. It'll return an A reference, which you can cast to the appropriate type. If you really need to be able to return a B or C from an ArrayList, you're going to have to keep track of two ArrayLists.
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    Default Re: ArrayList implementation

    Quote Originally Posted by KevinWorkman View Post
    It won't. It'll return an A reference, which you can cast to the appropriate type. If you really need to be able to return a B or C from an ArrayList, you're going to have to keep track of two ArrayLists.
    But this means I need to implement MyArrayList two times for exactly the same functionality. Is there no other way?

    I believe type casting indicates a bad design...

  6. #6
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    Default Re: ArrayList implementation

    Why do you need to extend ArrayList in the first place?

    If you need to keep track of two separate types (as opposed to simply keeping track of a single parent type), then you either need two ArrayLists or you need to cast. Simple as that. You said you only need the methods from class A. So why do you care whether something is a B or C?
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    Default Re: ArrayList implementation

    When adding an element to the ArrayList I know that it is either a B or a C. Before actually adding it I need to check if that B or C is already contained by the ArrayList. So ArrayList.add() which I want to override must invoke a ArrayList.contains or something.

    Hmm... looking at the documentation of ArrayList the method add may not return false. So I can not know if it is added or not.

    I think I need to think a little more about the design.

  8. #8
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    Default Re: ArrayList implementation

    Why don't you just use a Set?
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    Default Re: ArrayList implementation

    I moved responsibility from the list to the B and C class. Now the list does not need to know whether the A is in fact a B or C. So this question is no longer relevant.

    Thanks for your help Kevin.

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